A number of traditional rotating field devices, particularly d.c. motors have been of the type having a permanent magnet (PM) stator rotating armature brush commutator design. In recent years, materials and electronic devices have become available which make possible the construction of permanent magnet rotating field devices in which the magnetic material is contained in the rotor and the electric fields which spin the motor are generated by stator windings surrounding the rotor. Since device efficiency is largely a function of the volume of copper in the windings, the extra copper contained in the stator region of such devices gives this design approach an efficiency advantage for the same size and weight.
In these devices, the electric current is supplied to the spinning windings by sliding contacts on the shaft (called the commutator). As the shaft turns, the direction of the electric current in the windings is automatically switched at the proper time by the placement on the shaft of pads which receive the brush contacts. In the case of brushless devices, the permanent magnet rotates and the windings are stationary so that the switching of the electric current at the proper time to cause the rotor to spin must be accomplished in another way. This is done by electronically switching the electic current supplied to the windings in response to sensing the position of the rotor, usually through the use of optical or magnetic sensors or by analysis of the back e.m.f. waveforms.
FIG. 1 is a sectional radial view showing the configuration of a state of the art PM brushless device such as a dc motor. There is shown a magnet rotor 12 surrounded by a yoke 14. The inner surface of the yoke is proximal to the outer surface of the rotor except that there is an array of slots 16 extending parallel to the axis of the motor in which the windings 18 are captured. The inner face of the yoke 14 is as close as feasible to the surface of the rotor 12 ending in a foot 20 that forms an arc concentric with the radial surface of the rotor magnet 12. The space between the feet 20 and rotor magnet 12 is called the "gap" and the construction is intended to concentrate the flux from the energized windings in the gap.
The flux in the gap of the conventional PM dc motor is, ideally, fixed by the magnet rotor 12 and is little affected by the armature currents. The slots 16 generate what is called "reluctance" torque due to the alternating attraction and repulsion forces between the rotor poles and the slots, causing a ripple in the motor torque.
FIG. 2 shows the form of the magnetic field flux pattern for a conventional PM rotating field device such as a dc motor. Only the left hand side of the pattern is diagramed - - - a diagram of the flux pattern of the right hand side would be a mirror image. The walls of the slots 16 form a "magnetic" circuit which defines a highly permeable path for the concentration of magnetic flux.
There are a number of distinct advantages for rotary field devices such as brushless PM motors. The electric energy supplied to the motor is (primarily) converted into kinetic energy (shaft rotation) and heat (friction and electric losses). Greater efficiency means that more of the electrical energy is converted into kinetic energy and less into heat. Radio frequency noise from brushes is absent, making the motor more interference friendly. Brushes wear out, necessitating maintenance, and also spark, making them dangerous to use in explosive atmospheres. At large rpm's the brushes tend to "fly" over the contacts, thereby increasing the contact resistance, which generates heat, limits the performance of the motor and accelerates the wear on brushes. The brushless motor has none of these disadvantages and therefore has more power, is more reliable and lasts longer than its brush commutated counterpart.
A major source of energy loss in rotating field devices such as a motor occurs in the yoke due to induced or "eddy" currents caused by the changing magnetic fields. To reduce this loss, the yokes are conventionally formed as a stack laminated from many patterns stamped from thin metal sheet. The laminations electrically insulate the sheets thus keeping the eddy currents from propagating in the induced direction. This process of stamping and lamination is one of the more costly steps in the manufacture of the conventional motor.
The development, starting in the mid-1980's, of permanent magnet materials formed of neodymium-iron-boron provides matrials having an extremely high energy product (BH.sub.max). Recently, grain oriented material with energy product values of 30-47 MGOe (mega-gauss-oersteds) have become generally available.
The primary disadvantage of brushless motors is the extra expense and technology required in the electric commutation of the windings.